Abstract: With the hierarchy diagnostic model (HDM) presented by the authors, an approach to fault diagnosis of automobile engines is described, and a diagnostic expert system for automobile engines--AEDES is presented. Based on HDM, knowledge of expertise, wave signal understanding, and structure and function analysis of a diagnosed object is utilized in the system. By means of man-machine interaction, the system can identify the sources and causes of a failure. Now a prototype of the system has been completed. It has the functions of diagnosis, acquisition of several kinds of knowledge as well as a user-friendly Chinese language man-machine interface.